Telescopes are often used to concentrate light on detectors, such as photometers, spectrometers, bolometers, and the like. Incoming light in a direction that is not parallel to the axis of the telescope can degrade the quality of the desired image, if such off axis light is not suitably intercepted.
It is common practice to provide baffles for telescopes operating in the visible light wavelength region, which baffles are coated with "optical blacks". Off axis rays striking these surfaces are essentially absorbed; only a very small fraction is scattered. For such telescopes, the state of the art is highly developed and the literature is extensive.
More recently, there has been an increasing interest in telescopes for placement in earth orbit intended to operate in the infrared region, e.g., ranging in wavelength from 1 to 1,000 microns, i.e., 10.sup.-6 to 10.sup.-3 meters. For such telescopes, optical blacks may fail to absorb incident radiation adequately. Additionally, such coatings can become detached from the baffles, causing contamination of the telescope. The delicate nature of the optical black surfaces also makes refurbishment difficult.
With respect to optical systems that are cooled, such as by liquid helium or other coolant, for increasing imaging resolution of the optical system, absorption of radiation at infrared wavelengths constitutes a major heat load for the cooling system, requiring large, massive helium tanks for adequate cooling.
For these and similar reasons, attention has been directed recently to the use of reflective baffles in optical systems. For example, Davis, U.S. Pat. No. 3,488,103, issued Jan. 6, 1970 discloses a reflecting baffle having a concave, elliptical surface facing the direction off axis rays enter an optical system. However, in order for the system there disclosed to exclude off axis radiation sufficiently, the field of view of the optical system must be substantially reduced. Radovich, U.S. Pat. No. 4,217,026, issued Aug. 12, 1980, discloses an optical system incorporating a plurality of baffles that are each also concave with respect to the incident radiation to be reflected. This system also results in substantial reduction of the field of view, and it is only partially effective for rejecting skew off axis rays. As a result, approximately 10% of the incident off axis radiation is not reflected back out of the system. A similar system is described by Rock et al, "Use of Reflective Baffles for Control of Aperture Heat Loads and Stray Radiation," Optical Systems Engineering, Proceedings of SPIE, Vol. 330, pp. 60-65, January 1982, in which radiation absorbing surfaces are placed in the vicinity of the first baffle, in order to prevent skew rays from reaching the image plane. Such a system is also disclosed by Bremer, "Baffle Design for Earth Radiation Rejection in the Cryogenic Limb Scanning Interferometer/Radiometer", Optical Engineering, Vol. 22, No. 1, pp. 166-171, January-February 1983.
A variety of other approaches are also shown in the following patents for preventing off axis rays from degrading an image produced by an optical system: Taylor, U.S. Pat. No. 2,738,700, issued Mar. 20, 1956; Nicoll, U.S. Pat. No. 2,821,109, issued Jan. 28, 1958; Lawrence, U.S. Pat. No. 3,016,798, issued Jan. 16, 1962; Buttweiler, U.S. Pat. No. 3,648,056, issued Mar. 7, 1972; Vulmiere et al, U.S. Pat. No. 3,675,984, issued July 11, 1972; McCracken, U.S. Pat. No. 3,905,675, issued Sept. 17, 1975. While the art relating both to absorption and reflection of off axis rays to prevent image degradation in optical systems is therefore a well developed one, a need still remains for further development of such systems, in order to meet the stringent demands of infrared telescopes and similar optical systems.